1. Field of the Invention
The present invention relates to a highly reliable solar cell module having an improved surface side covering material with a specific nonwoven glass fiber member. More particularly, the present invention relates to a highly reliable solar cell module having an improved surface side covering material with a specific nonwoven glass fiber member comprising a nonwoven glass fiber member having a texture bonded with a resin binder or a nonwoven glass fiber member treated with a silane coupling agent, which excels in exterior appearance, resistance to scratching (hereinafter referred to as scratch resistance), inflammability, weatherability, light resistance, and heat resistance.
2. Related Background Art
In recent years, the societal consciousness for the problems relating to the environment and energy has been increasing all over the world. Particularly, global-warming because of the so-called greenhouse effect due to an increase of atmospheric CO.sub.2 has been predicted to cause a serious problem. In view of this, there is an increased demand for a means of power generation capable of providing clean energy without causing CO.sub.2 buildup.
Now, public attention has been focused on solar-cells capable of serving as a non-exhaustable power generation source for supplying electric power without causing global-warming while meeting demand.
In order to use a solar cell as a power generation source, it is usually designed into a solar cell module in a desired configuration which can be used as the power generation source. Such a solar cell module has been widely used as the power generation source by installing it, for instance, on the ground or on a roof of a building.
Now, there are known solar cell modules in which an amorphous silicon solar cell element is used. Such a solar cell module is usually provided with a light transmissive surface side covering material on the light incident side face to protect the solar cell element. The surface side covering material is required to have (1) sufficient transparency for visible light used in solar cell power generation, (2) sufficient resistance to scratching to prevent the solar cell element enclosed in the solar cell module from suffering from externally applied stresses such as external scratching, external shock, and the like, (3) sufficient weatherability to protect the solar cell element without deteriorating use outdoors, and (4) sufficient non-flammability such that the surface side covering material itself is hardly burned.
It is known to use a glass member as an outermost surface covering material of a solar cell module.
In the case where the glass member is used as the outermost surface covering material of a solar cell module in which an amorphous silicon solar cell element is used (this solar cell module will be hereinafter referred to as amorphous silicon solar cell module), because the glass member is heavy, poor in flexibility and costly, the advantages of the amorphous silicon solar cell element, in that the amorphous silicon solar cell element is light a flexible and inexpensive, cannot be fully used. For this reason, as the outermost surface side covering material of the amorphous silicon solar cell module, a transparent thin film of a fluoride polymer is used. In this case, a transparent organic polymer such as EVA (ethylene-vinyl acetate copolymer) or the like as a filler is usually disposed inside the transparent thin film as the outermost surface side covering material. As the filler, it is usually used such a transparent organic polymer which is less expensive and which therefore can be used in a large amount in order to seal the amorphous silicon solar cell element in the amorphous silicon solar cell module. A preferable example of the transparent organic polymer used as the filler is said EVA which excels in heat resistance and weatherability.
The above, mentioned fluoride polymer thin film used as the outermost surface covering material excels in weatherability and repellency. In addition, the use of the fluoride polymer thin film as the outermost surface covering material provides an advantage to diminish the reduction of the photoelectric conversion efficiency of the solar cell module because of a reduction in the light transmittance due to resin's yellowing or clouding based on deterioration of the resin or due to surface stain. It is also an advantage that it is possible to attain an amorphous silicon solar cell module excelling in flexibility.
In the case of using a resin film such as a fluoride polymer thin film or the like as the outermost surface covering material of a solar cell module, the scratch resistance thereof is inferior to that of a solar cell having an outermost surface covering material comprising a glass member. In order to eliminate this problem, a nonwoven fiber member such as a nonwoven glass fiber member is often impregnated into a resin used as a filler.
Now, to use a nonwoven glass fiber member comprising a CRANE GLASS 230 (trademark name) together with a binder comprising a vinyl alcohol resin in an amount of 10 wt. % or more as a constituent of a surface covering material (including a glass member situated at the outermost surface side) of a solar cell module is described, for example, in Annual Report "Investigation of Test Methods, Material Properties, and Processes for Solar Cell Encapsulant", page 10-1, June 1979 (published by U.S. Department of Energy), or in Final Report on the Investigation of Proposed Process Sequence for the Array Antomated Assembly task, page 233, August 1980 (published by U.S. Department of Energy) (these will be hereinafter referred to as Reference 1).
The use of the nonwoven glass fiber member in Reference 1 is aimed not at improving the resistance to scratching of the solar cell module but principally at (a) ensuring a certain distance between the solar cell element and the glass member, (b) electrically isolating the solar cell element from the outside, and (c) attaining a pathway for deaeration in the vacuuming process in the production of the solar cell module.
Further, Japanese Unexamined Patent Publication No. 1875/1985 (hereinafter referred to as Reference 2) discloses the use of a glass fiber member by impregnating it into a filler upon the production of a solar cell module using a plurality of solar cell elements and a glass member as the outermost surface covering material by way of lamination process, in order to solve problems in that in the lamination process, the cell elements are moved to cause mutual contact between adjacent cell elements, or their strings are moved to externally raise through a glass member situated on the outermost surface side.
Further in addition, Japanese Patent Publication No. 33756/1987 (hereinafter referred to as Reference 3) discloses a solar cell module covered by a surface protective film of a transparent organic polymer resin as its outermost surface layer, which is produced by arranging two glass fiber members having slight end portions on opposite surfaces of a photovoltaic element and pouring an organic polymer resin on the resultant.
However, the prior art disclosed in References 1 to 3 have problems as will be described below.
In the case of Reference 1, the layer constituted by the nonwoven glass fiber member (the CRANE GLASS 230) and the vinyl alcohol binder in an amount 10 wt. % or more is readily markedly colored in an atmosphere with high temperature, resulting in reducing the photoelectric conversion efficiency of the solar cell module.
In the case of Reference 2, because the glass member is used as the outermost surface covering material, in the case of a solar cell module in which an amorphous silicon solar cell element is used, the advantages of the amorphous silicon solar cell element in that it is light and flexible cannot be fully used, as previously described.
In the case of Reference 3, since one glass fiber member is disposed on the light receiving face side of the photovoltaic element as above described, in order for the surface side covering material of the solar cell module to have sufficient scratch resistance, it is necessary to increase the amount of the organic polymer resin used or thicken the thickness of the glass fiber member. However, to increase the amount of the organic polymer resin entails a problem in that the surface side covering material of the solar cell module becomes inferior in non-flammability.
To thicken the glass fiber member causes a problem in that the glass fiber member is often raised to expose to the outside through the surface protective film when the solar cell module is continuously used in outdoors over a long period of time. Particularly, the portion of the glass fiber member which is extended to externally expose through the surface protective film is free of a transparent organic polymer resin as an adhesive, and the surface protective film and a portion of the photovoltaic element situated under said portion of the glass fiber member is free of a transparent organic polymer resin as a filler. Therefore, interfacial portions of the solar cell module where said portion of the glass fiber member is involved are insufficiently adhered. Because of this, moisture is liable to invade into the inside of the solar cell module, resulting in deteriorating the characteristics of the photovoltaic element and a leakage current is caused through the moisture.
Hence, the solar cell module according to Reference 3 is problematic in terms of reliability when it is continuously used outdoors over a long period of time, specifically over about 20 years from the viewpoint that the lifetime of a solar cell module as a power generation source is generally considered to be 20 years. Further, recently, there is a demand for a solar cell module as a power generation source to have a 50 year lifetime outdoors. The solar cell module according to Reference 3 apparently cannot meet this demand.
A solar cell module having an outermost surface covering material comprising an organic polymer resin film is more likely to have problems due to moisture in comparison with the case of a solar cell module having an outermost surface covering material comprising a glass member. Said troubles include that the organic polymer resin film as the outermost surface covering material is liable to allow moisture to permeate therethrough into the inside of the solar cell module, and that moisture invades through pinholes of the organic polymer resin film into the inside of the solar cell module. The latter problem is serious in that in the case where the moisture invaded contains an electrolyte, when it reaches the solar cell element, the electrical insulation between the solar cell element and the outside is broken to leak electrical current to the outside.
Incidentally, for (i) a conventional solar cell module (of 3600 cm.sup.2 in area) comprising a solar cell element sealed by an organic sealing resin and a surface side covering material comprising an organic polymer resin film as an outermost surface protective film an& a nonwoven glass fiber member situated under the outermost surface protective film in which an organic sealing resin is present between the organic polymer resin film and the nonwoven glass fiber member and (ii) another conventional solar cell module (of 3600 cm.sup.2 in area) comprising a solar cell element and a surface side covering material comprising an organic polymer resin film as an outermost surface protective film but having no nonwoven glass fiber member in which an organic sealing resin is present between the organic polymer resin film and the solar cell element, the present inventors observed electric insulation resistance between the outermost surface protective film and the solar cell element of each solar cell module in the following manner. For each solar cell module, the initial electric insulation resistance between the outermost surface protective film and the solar cell element was measured. Then, the surface covering material of each solar cell module was immersed in service water for 32 days, during which the electric insulation resistance between the outermost surface protective film and the solar cell element was measured after the immersion in the service water for 2 days, 4 days, 8 days, 16 days, and 32 days. The measured electric insulation resistances for each solar cell module are collectively shown in Table 1.
From the measured results shown in Table 1, it is understood that in the case of the solar cell module (i) with the nonwoven glass fiber member, the electric insulation resistance between the outermost surface protective film and the solar cell element is markedly reduced as the water immersion period is increased. One of the reasons for this is considered to be that moisture which invaded through pinholes of the outermost surface protective film passes through the interface between the organic sealing resin and the nonwoven glass fiber member to reach the solar cell element.
In the case of the solar cell module (ii) with no nonwoven glass fiber member, it is understood that the reduction in the electric insulation resistance between the outermost surface protective film and the solar cell element is slight. This situation proves that the moisture invasion pathway in the case of the solar cell module (i) lies in the interface between the organic sealing resin and the nonwoven glass fiber member.